water content measurement in building barriers and materials using

water content measurement in building barriers and materials using

Proceedings of ECOpole
Vol. 2, No. 1
2008
Zbigniew SUCHORAB1, Agnieszka JEDUT1 and Henryk SOBCZUK1
WATER CONTENT MEASUREMENT IN BUILDING BARRIERS
AND MATERIALS USING SURFACE TDR PROBE
POMIARY WILGOTNOŚCI MATERIAŁÓW ORAZ PRZEGRÓD
BUDOWLANYCH Z ZASTOSOWANIEM POWIERZCHNIOWEJ SONDY TDR
Summary: Water contained in external walls is one of the the basic factors restricting the functioning of
buildings. Its negative influence should be evaluated both in the constructional and hygienic aspects. It is caused
by the fact that water is not only the reason of successive destruction of buildings’ construction, but also composes
the base for the growth of microorganisms and moulds. Such problem is typical for the buildings without moisture
control and monitoring and causes the respiratory system illnesses, infections, allergies, eyes and skin
sensitisations. The buildings affected by the problem of moisture in most cases are stricken with the Sick Building
Syndrome, which is caused by the use of not human-friendly materials, defective ventilation or high moisture
previously mentioned. Water contained in the external walls significantly diminishes their thermal characteristics,
which induces the increased heat losses in cold season, reduction of perceptible temperature and thermal comfort
of accommodations. All the previously mentioned negative aspects of water influence on the buildings cause the
need to find the precise, user-friendly method of water content valuation in the walls. One of them is TDR (Time
Domain Reflectometry). This technique bases on the measurement of the electromagnetic pulse propagation in
examined medium. The dielectric constant of the material (determined with the TDR device) is the base for its
moisture estimation. The TDR method has got a lot of advantages (high monitoring potential, insensitivity to the
salinity, relatively simple service) and has been used in moisture measurements of porous materials, especially of
the soils, for many years. By now, this technique has not found the common implementation in the building area
which is caused by its invasive character - it requires the installation of the steel rods in the examined medium,
which sets many problems in case of building materials and envelopes.The aim of this paper is to propose the
alternative idea of the TDR probe - surface probe, which enables the moisture measurements of hard building
materials and envelopes. For these materials, the use of classical probe is difficult to realize, because of problems
with the introduction of the steel rods in the examined medium. This, modified, TDR method enables the effective
moisture measurements without the need to destroy the building barriers structure.
Keywords: building materials, building barriers, building envelopes, moisture, reflectometric moisture
measurement methods, dielectric methods, Time Domain Reflectometry, surface TDR probe
The TDR (Time Domain Reflectometry) methodology was first applied in electronics
for electric cables discontinuities detection. It was realized by measurement of the
electromagnetic signal propagation velocity. Application of that idea in a reverse mode is
the base of functioning of TDR moisture measurement method. With the known
transmission line length (rods of the probes), measurement of the signal propagation
velocity enables the determination of the dielectric constant value of the surrounding
material (porous medium). Dielectric constant is an electrical parameter of materials, which
in case of porous media, strongly depends on water content [1]. For almost 20 years the
TDR method has been applied for moisture determination of soils [2] and has been
continuously developed by introducing better calibration methods [3] or technologies. Soils
are characterized by loose structure and the invasive measurements with typical TDR
probes construction have not been problematic. Figure 1 presents typical constructions of
TDR probes built of concentric cable, head and steel rods - usually 2.
1
Faculty of Environmental Engineering, Lublin University of Technology, Nadbystrzycka 40B, 20-618 Lublin,
tel./fax +48 81 538 49 97, email: [email protected]
124
a)
Zbigniew Suchorab, Agnieszka Jedut and Henryk Sobczuk
b)
Fig. 1. Classical TDR probes (Easy Test/ Lublin): a) for laboratory measurement of water content (LP/ms), b) for
field water content measurement (LP/mts)
Fig. 2. TDR reflectogram for laboratory LP/ms probe construction. Upper curve indicates dry material, middle intermediate states and finally bottom - water (with the highest value of dielectric constant)
The typical TDR probe is an extension of a concentric cable where the core separates
from screening and they both transform into the steel rods. Part of these rods can be closed
in a small resin or plastic cover. The beginning of the probe and also its termination are the
concentric cable discontinuities. The TDR device receives the echo of the electromagnetic
signal returning from the probe in the form of reflectogram (Fig. 2), and the discontinuities
are represented by peaks. The distances between two main peaks vary because of the
different dielectric constants (due to water content change).
Water content measurement in building barriers and materials using surface TDR probe
125
Modifications
As it was mentioned above, the application of clasical TDR probes is nearly impossible
for rocks or most of building materials which is mainly caused by problems of steel rods
installation. This problem can be solved by application of the TDR surface probes, which
can be used for moisture determination of building materials or barriers without internal
rods installation. The surface probe construction for non-invasive moisture determination
was previously proposed by Sobczuk [4] and Wraith et al. [5] and some prototypes are
presented in Figure 3.
a)
b)
Fig. 3. TDR surface probes: a) for building materials moisture measurement [4], b) TDR surface probes for soil
moisture measurement [5]
The probes are made of plexi (Fig. 3a) with aluminium angle bars as measuring
elements. Communication with TDR device provides BNC interface.
Experiment
Experimental setup consisted of the TDR device - FOM/mts (Field Operated
Multimeter - Easy Test/ Lublin), PC computer as a control station (connected with FOM via
RS-232c interface) and concentric cable for surface probes connection.
Application of unusual TDR probes required modification of TDR device handling;
standard operation of FOM TDR device requires only pressing of buttons and reading out
the moisture, or eventually dielectric constant - easily recalculated into moisture with Topp
[2] or Malicki’s [3] formulas.
For surface TDR probes a PC computer was used which provided widespread control
over the measuring device, and allowed the collection of full reflectograms (not only the
simple readouts of the moisture).
Three samples of AAC (aerated autoclaved concrete) were prepared for examination.
They were dried out in the temperature of 105ºC. Surfaces of the samples were polished to
minimize the eventual measurement errors. First sequence of measurements was made on
dry samples. Together with TDR, a sequence of gravimetric measurement was done (to
compare the reflectometric readouts with the gravimetric ones). Then the samples were
moistened and another sequence of measurements was done. Each cycle was repeated
several times until the samples of AAC were saturated
126
Zbigniew Suchorab, Agnieszka Jedut and Henryk Sobczuk
Results discussion
For each water content of the sample a reflectometric measurement was done.
Diagrams below represent the TDR readouts from the surface probe with different water
contents. Two first peaks in each diagram are not important from the point of the
measurement and are skipped in this analysis. It is clearly visible that first important,
positive peak is located in the same place of each reflectogram and it means the beginning
of the probe. The second one is shifted to right in each reflectogram which means that the
signal needed more time to return from the end of the probe. The velocity of the signal
changed (slowed down) because of the rise of the dielectric constant. It was caused by
increasing water content in the examined AAC samples.
Fig. 4. TDR reflectograms from the surface probes
Conclusions
TDR technology is a good option for moisture measurement of the porous media. The
major advantages of the method are the high monitoring potential and good accuracy.
The most important factor that limits application of that technology is its invasive
character, especially for the building materials and barriers. It seems that introduction of
surface TDR probes has a high potential in moisture measurements. It enables to avoid the
problems of probes installation and having a good calibration methodology we may be able
to conduct laboratory and field measurements in hard building materials or rocks.
References
[1]
[2]
[3]
[4]
[5]
Curtis H.L. and Defandorf F.M.: Dielectric constant and dielectric strength of elementary substances, pure
inorganic compounds, and air. E.D. Washburn (Ed.). International Critical Tables of Numerical Data,
Physics, Chemistry and Technology. McGraw-Hill, New York 1929, 6, 73-107.
Topp G.C., Davis J.L. and Annan A.P.: Electromagnetic determination of soil water content: Measurements
in coaxial transmission lines. Water Resour. Res., 1980, 16, 574-582.
Malicki M.A., Plagge R., Renger M. and Walczak R.T.: Application of time-domain reflectometry (TDR) soil
moisture miniprobe for the determination of unsaturated soil water characteristics from undisturbed soil
cores. Irrigation Sci., 1994, 13, 65-72.
Sobczuk H.: Sonda do pomiaru wilgotności ośrodków porowatych, zwłaszcza materiałów budowlanych,
Polska. Zgłoszenie patentowe nr 350713, 19-05-2003
Wraith J.M., Robinson D.A., Jones S.B. and Long D.S.: Spatially characterizing apparent electrical
conductivity and water content of surface soils with time domain reflectometry. Comput. Electron. Agricult.,
2005, 46, 239-261.
Water content measurement in building barriers and materials using surface TDR probe
[6]
127
Suchorab Z., Sobczuk H., Łagód G., Pavlik Z. and Cerny R.: Zastosowanie metody TDR do pomiaru
wilgotności materiałów budowlanych, materiały. Monografie Komitetu InŜynierii Środowiska PAN, 2005c,
2(33), 1063-1070.
POMIARY WILGOTNOŚCI MATERIAŁÓW ORAZ PRZEGRÓD
BUDOWLANYCH Z ZASTOSOWANIEM POWIERZCHNIOWEJ SONDY TDR
Streszczenie: Woda zawarta w zewnętrznych przegrodach budowlanych naleŜy do podstawowych czynników
ograniczających funkcjonowanie budynków. Jej negatywny wpływ na obiekty naleŜy oceniać zarówno ze
względów konstrukcyjnych, jak i higieniczno-sanitarnych. DuŜa wilgotność przegród budowlanych jest przyczyną
sukcesywnego niszczenia konstrukcji budynków (krystalizacja soli, wielokrotne procesy zamarzania
i rozmarzania w okresie zimowym, rozkład drewna oraz przyspieszona korozja stalowych elementów
zbrojeniowych). Woda, w sposób pośredni, negatywnie wpływa na środowisko wewnętrzne pomieszczeń, tworząc
podłoŜe do rozwoju szkodliwych mikroorganizmów oraz grzybów pleśniowych. Jest to problem typowy dla
obiektów z nieuregulowaną i niemonitorowaną wilgotnością przegród, będący przyczyną chorób dróg
oddechowych, infekcji, alergii oraz podraŜnień oczu i skóry. Obiekty dotknięte problemem zawilgocenia przegród
zewnętrznych w większości przypadków określamy jako dotknięte zespołem chorego budynku SBS (Sick Building
Syndrome), którego przyczyną jest zastosowanie nieprzyjaznych człowiekowi materiałów budowlanych, wadliwa
wentylacja lub właśnie nadmierne zawilgocenie przegród. Woda w przegrodach budowlanych
w znaczący sposób obniŜa ich charakterystyki cieplne, co w konsekwencji prowadzi do zwiększonych strat ciepła
w sezonie grzewczym, obniŜenia temperatury odczuwalnej, obniŜenia komfortu cieplnego pomieszczeń.
Wszystkie wyŜej przedstawione negatywne aspekty wpływu wody na obiekty budowlane stwarzają potrzebę
znalezienia precyzyjnej i moŜliwie łatwej metody oceny zawartości wody w przegrodach. Do takich metod
zaliczamy reflektometryczną metodę pomiaru wilgotności TDR (Time Domain Reflectometry). Funkcjonowanie
tej techniki oparte jest na pomiarze prędkości propagacji impulsu elektromagnetycznego w badanym materiale.
Wyznaczona ze znanej zaleŜności względna stała dielektryczna materiału jest podstawą do ustalenia jego
wilgotności. Metoda ta posiada wiele zalet (moŜliwość ciągłego monitoringu, brak wraŜliwości na zasolenie,
stosunkowa prostota obsługi) i od wielu lat stosowana jest do pomiaru wilgotności ośrodków porowatych,
a w szczególności ośrodków gruntowych. Nie znalazła ona do tej pory szerokiego zastosowania w dziedzinie
budownictwa. Przyczyną tego jest jej inwazyjny charakter - do realizacji pomiaru niezbędne jest wprowadzenie
stalowych prętów w badany ośrodek, co stwarza wiele problemów w przypadku materiałów oraz przegród
budowlanych. Celem pracy jest przedstawienie alternatywnej konstrukcji - powierzchniowej sondy TDR, która
umoŜliwia pomiary wilgotności materiałów oraz przegród budowlanych, charakteryzujących się znaczną
twardością, dla których zastosowanie klasycznej, dwuprętowej sondy, wymagającej wprowadzenia stalowych
prętów w badany ośrodek, jest trudne do zrealizowania. Metoda ta umoŜliwia skuteczne pomiary wilgotności bez
konieczności niszczenia konstrukcji przegrody.
Słowa kluczowe: materiały budowlane, wilgotność przegród budowlanych, reflektometryczne metody pomiaru
wilgotności, metody dielektryczne, Time Domain Reflectometry, powierzchniowa sonda TDR